This invention relates to extruders and injection machines of the type in which a screw rotatable within a barrel is employed to extrude or inject molten resinous material downstream to an outlet port of the barrel. More particularly, this invention is concerned with thorough mixing and melting of resinous material using a special cylindrical apparatus in the screw arrangement.
A plasticating device or unit commonly used today receives polymer or thermoplastic resin pellets, granules or powders, then heats and works the resin to convert it into a melted or molten state. The melt or molten material is delivered under pressure through a restricted outlet or discharge port to make the finished article. It is desirable that the molten material or extrudate leaving the plasticating device be homogeneously mixed, completely melted, and uniform in temperature, viscosity, color and composition.
More specifically, the basic plasticating device includes an elongated cylindrical barrel which is heated at various locations along its length. A screw extends longitudinally through the barrel. The screw has a core with a helical flight thereon and the flight cooperates with the cylindrical inner surface of the barrel to define a helical valley for forward passage of the resin to the plasticating device outlet port.
Since there are several different types of thermoplastic resins or polymers, and with each having different physical properties and characteristics, there are different screw configurations. In general, however, the typical plasticating screw has a plurality of sections along its extended axis with each section being designed for a particular function. Ordinarily, there is a feed section, a transition section and a metering section in series.
The feed section extends forward from a feed opening where solid thermoplastic resins, in pellet, granular or powder form, are introduced into the plasticating unit and pushed forward by the screw along the inside of the barrel. The resin is then worked and heated in the transition section so that melting occurs. After approximately 40 to 80 percent of the resin has been melted, solid bed breakup occurs, and solids become randomly dispersed within the melt. It is important to note that most melting initially occurring in the transition section takes place at or near the heat source of the barrel. Then, melting becomes enhances as solids subsequently become dispersed within the melt. To assure a homogeneous melt, therefore, it is important that the transition section enhances turbulent flow, as opposed to laminar flow, so that all the resin comes within the heating vicinity of the barrel or is dispersed within the melt. Otherwise, the presence of minute unmelted resin particles will appear in the finished article.
The transition section has a reduced root depth of the helical passageway, as compared with the feed section, to reflect the volume reduction due to melting of the feed. The transition section leads to the metering section. The metering section, as one of its intended functions, provides a constant flow of molten material toward the outlet port. In addition, it is important that the metering section melt any unmelted solids and mix and maintain the molten resin in a homogeneous and uniform composite.
As described in U.S. Pat. No. 3,486,192, controlled melting and mixing of thermoplastic materials can be enhanced with a special cylindrical or tapered cylindrical shearing means in the transition section of the plasticating screw arrangement. Input grooves are arranged to extend substantially longitudinally in the surface of the special module or section with groove openings at the input and that terminate in dead ends before reaching the output end of the section. Output grooves are also arranged between the input grooves extend substantially longitudinally in the surface of the special section with openings at the output end and which terminate in dead ends before reaching the input end of the section. As explained in U.S. Pat. No. 3,486,192 to Le Roy, as the special section rotates, plastic material is fed under pressure from a prior stage into and along the input grooves before shearing over lands between the grooves and passing into the output grooves, through the output openings and into the next conveyor stage.
Although this configuration may satisfy many general needs, thermal and chaotic mixing can be improved even more for various thermoplastic resin and polymer materials by having a special cylindrical or tapered cylindrical shearing module that allows for a continuous back flow of molten material. The back flow of molten material gets recycled through portions of the transition section and is remixed with newly advancing molten resin, thereby further assuring the uniformity of the composition, viscosity, color and temperature of the molten resin leaving the plasticating device.
The present invention is directed to an improvement in the screw configuration, plasticating device and method for improving mixing and melting of resin material by a screw having a helical flight disposed within and cooperating with the inner-wall of a heated barrel to form an upstream where material is fed into and a downstream where material exits the device. In accordance with this invention, a special cylindrical module is included in the screw arrangement. The cylindrical module of the instant invention comprises a helically cut channel traversing in a reverse helical direction compared with the forward flow direction caused by the flight in the prior downstream stage. With this invention, a plurality of discrete grooves are arranged in a discontinuous helix cut.
More specifically, this invention includes input grooves extending in the surface of the cylindrical module with input groove openings beginning within the helically cut channel and terminating in a dead end before intersecting the immediately neighboring channel upstream. Output grooves, being adjacent to the input grooves, extend in the surface of the module with output groove openings beginning within the helically cut channel and terminating in a dead end before intersecting the immediately neighboring channel downstream. A barrier or land is between the pair of adjacent output and input grooves to prevent direct communication therebetween and to create a shearing effect of material passing thereover.
Although a plurality helix cut paths of both input and output grooves are employed in the preferred embodiment of the instant inventive module, it is also within the scope of the invention to use one path of discontinuous helix cut input and output grooves. Also, the clearance between the inner wall of the plasticating barrel and barriers between input and output grooves of the instant invention may be adjusted and/or varied as dictated by operating conditions.
In summary, the helically cut channel traversing in a reverse helical direction in the present invention allows for the back flow of molten material which is recycled therethrough. Also, input and output grooves cooperate with the barriers therebetween to enhance shearing and turbulent flow as pressure from the prior upstream stage forces plastic through the special module and into the next conveyor stage.
In terms of the method, the present invention relates to plasticating resinous material in a molten state under pressure by feeding resinous material in a solid state to the feed section of a screw plasticating device, such as an extruder or injection molding machine. As the screw is rotated in a cylindrical barrel having an inner surface, the flight of the screw and the inner surface cooperate to force and move resin material along a helical path forward toward the outlet port. In the transition section, heat is applied to the barrel which transfers to the material while working the material between the barrel and the screw. As a result of heat and mixing, the material is converted into a molten state.
To enhance mixing and melting, the instant invention uses a special cylindrical module included in the plasticating screw arrangement. More specifically, the instant cylindrical module has a helically cut channel traversing in a reverse helical direction, as compared with the forward direction of the flight in the prior stage. Input grooves and output grooves extend in the surface of the special cylindrical module with barriers therebetween as previously described, for creating turbulent flow, mixing, shearing and recyling of material therethrough.
Many other objects and features of the present invention will be obvious to those of skill in the art upon contemplation of the entire disclosure herein in connection with the accompanying drawings.